In gas turbine engines, fuel is burned within a combustion chamber to produce hot gases of combustion. The gases are expanded within a turbine section producing a gas stream across alternating rows of stationary stator vanes and turbine rotor blades to produce usable power. Gas stream temperatures at the initial rows of vanes and blades commonly exceed 2,000.degree. F. Blades and vanes, susceptible to damage by the hot gas stream, are cooled by air compressed upstream within the engine and flowed to the turbine components.
One substantial problem associated with such systems is the transfer of cooling air from stationary cavities within the engine stator to a rotor assembly for subsequent distribution to the interior of the rotor blades. For this purpose, the TOBI nozzle is a well-known device. In particular, an inlet of the TOBI nozzle receives compressed air emanating from the compressor and passes the cooling air through annually spaced passages that impart a swirling movement and directs the discharging stream of cooling air tangentially to the rotating turbine assembly.
The volume and direction of the cooling air are features of the effectiveness of its cooling capacity and its effect to the overall engine performance. It is important that only the correct amount of cooling air be utilized as any additional air could penalize efficiency of combustion while too little air would result in overheating of the turbine blades. Hence, ideally the TOBI nozzle will optimize the use of turbine cooling air.
A TOBI nozzle with adjustable air flow is disclosed in U.S. Pat. No. 4,708,588 entitled TURBINE COOLING AIR SUPPLY SYSTEM, issued to Schwarz et al on Nov. 24, 1987. According to this patent, the amount of cooling air flowable to the turbine blades of a gas turbine engine is made variable in response to the blade requirements by enabling the opening and closing of a portion of injectors of a tangential on-board injection system supplying cooling air to the turbine rotor assembly. Actuator means are provided to rotate an annular ring such that the orifices in the ring align with a number of injectors at conditions of maximum cooling flow requirements, and are in misalignment at conditions requiring a lesser air flow. The turbine cooling air supply system disclosed in this patent requires additional material weight for the annular ring and the actuator means, which are critical and sensitive to a gas turbine engine, particularly used in aircrafts. The control system for a dynamic air flow adjustment increases problems associated with operation stabilities. Overall, the manufacturing cost will increase significantly for such a dynamic adjustable system.
Decreasing the manufacturing costs is another concern. For this purpose, a simple structure for low cost fabrication is desirable. For example, U.S. Pat. No. 4,435,123, entitled COOLING SYSTEM FOR TURBINES issued to Levine on Jul. 2, 1985, discloses a TOBI nozzle fabricated as a unitary structure that is formed by investment casting and may be easily secured in position in the engine. However, the problem encountered with a unitary investment-cast TOBI nozzle is that the temperature differential encountered in proximity to the attachment structure adjacent the turbine stator vanes is more severe than the structural integrity of the casting can tolerate. To utilize the unitary unit, the attachment end of the unit would need to be made of a different material from the investment cast material so that it could tolerate the temperature stress limitations. Obviously, this would require a weldment of the TOBI nozzle which is not only expensive and difficult but it presents problems that should otherwise be avoided.
U.S. Pat. No. 4,526,511 entitled ATTACHMENT FOR TOBI, issued to Levine on Jul. 2, 1985, discloses an improved unitary structure using attachment means which are a shielding ring. The shielding ring is segmented into several pads, circumferentially spaced to permit thermal growth. This ring serves to shield the mounting flange from the high temperature air.